This invention relates generally to dissolving pulp by cooking and particularly with pre-hydrolysis and Kraft cooking of wood chips
Hydrolysis of comminuted cellulosic fiber feed material, such as wood chips, before Kraft cooking is describe in U.S. Pat. Nos. 3,3380,883 and 4,436,586, and in Blom et al, “Development of the Alva Prehydrolysis Process, Part Two: Mill Scale Application,” pp. 409-416 TAPPI Proceedings, 1981 Pulping Conference. Prehydrolysis is typically used to dissolve pulp for use in forming rayon or plastics.
The pseudolignins formed during hydrolysis of wood can coat wood chips and the surfaces of the reactor and other equipment exposed to the hydrolyzed cellulosic fiber feed material. The pseudolignins are formed through recondensation and polymerization of reactive byproducts from the hydrolysis of the cellulosic fiber feed material. These reactive components include dissolved hemicellulose and other wood compounds. Fragments of lignin that dissolve in hydrolysis can also coat the wood chips and the surfaces of the pre-hydrolysis reactor and other equipment. The pseudolignins and lignin fragments tend to build up on the surfaces of chips and equipment most when in an acidic environments; such as typically exists in pre-hydrolysis. The dissolved complex organic molecules in the pseudolignins and dissolved lignins may coat the chips and block pores in the chips. The pores should be open to allow penetration of alkali during the Kraft stage of the process. Buildups of pseudolignins and dissolved lignins on equipment can block the flow passages for the cellulosic feed material, interfere with rotation of moving parts and otherwise interfere with the operation of equipment.